1. Field of the Invention
The present invention relates to titanium-aluminium (Ti-Al)type lightweight heat-resistant materials, and particularly to Ti-Al type lightweight heat-resistant materials which are useful for the manufacture of various machine parts.
2. Description of the Prior Art
Recently, for realizing higher performance and higher efficiency of engines and the like, parts to be used for high-speed reciprocating movement, such as engine valves, pistons and rocker arms, or parts to be used for high-speed rotation, such as turbine blades and turbocharger rotors for gas turbines and jet engines, are required to be more and more lightweight and excellent in heat resistance. Therefore, researches and developments on materials used for these parts have been extensively carried on in order to meet such requirements.
At present, as materials for these parts, nickel(Ni)-base superalloys are predominantly used. Other materials used therefor are titanium alloys and ceramic materials. However, the Ni-base superalloys have a disadvantage in that they are heavyweight, and the ceramic materials have a disadvantage in that they are inferior in ductility and hence unreliable as materials for the above parts.
Ti-Al type alloys based on Ti-Al intermetallic compounds have recently been made much account of as a material for the above parts. The Ti-Al alloy is much lighter in weight in comparison to the Ni-based superalloys, and superior in ductility in comparison to the ceramic materials. However, the Ti-Al alloy has a disadvantage in comparison to the Ni-base superalloys and the ceramic in that the oxidation resistance of the Ti-Al alloy deteriorates at high temperature, above 800.degree. C. It has been found that the oxidation resistance of the Ti-Al alloy is improved by adding a combination of niobium (Nb) and silicon (Si).
The Ti-Al alloy containing Si/Nb has excellent specific tensile strength (strength/density) which is equal to that of a typical Ni-base superalloy such as Inconel 713C. However, the Ti-Al-Si-Nb alloy still has a disadvantage in that its ductility at room and high temperatures is low, making it brittle. Accordingly, it is desirable to improve the ductility of the Ti-Al-Si-Nb alloy.
While addition of manganese (Mn), chromium (Cr) or the like to the Ti-Al alloy has been contemplated to improve the ductility of the alloy at room temperature, there has been no development for improving the ductility of Ti-Al-Si-Nb alloy, thereby simultaneously improving the ductility and the oxidation resistance of the Ti-Al alloy.
Accordingly, it has been eagerly desired to develop Ti-Al-Si-Nb alloys having improved ductility and strength at room temperature and high temperature without impairing their excellent oxidation resistance.